1. Field of the Invention
The present invention relates to noble metal alloy catalysts, especially to platinum, palladium and ruthenium alloy catalysts, which are useful in fuel cell electrodes and other catalytic structures.
2. Description of Related Technology
A fuel cell is an electrochemical device for directly converting the chemical energy generated from an oxidation-reduction reaction of a fuel such as hydrogen or hydrocarbon-based fuels and an oxidizer such as oxygen gas (in air) supplied thereto into a low-voltage direct current. Thus, fuel cells chemically combine the molecules of a fuel and an oxidizer without burning, dispensing with the inefficiencies and pollution of traditional combustion.
A fuel cell is generally comprised of a fuel electrode (anode), an oxidizer electrode (cathode), an electrolyte interposed between the electrodes (alkaline or acidic), and means for separately supplying a stream of fuel and a stream of oxidizer to the anode and the cathode, respectively. In operation, fuel supplied to the anode is oxidized releasing electrons which are conducted via an external circuit to the cathode. At the cathode the supplied electrons are consumed when the oxidizer is reduced. The current flowing through the external circuit can be made to do useful work.
There are several types of fuel cells, including: phosphoric acid, molten carbonate, solid oxide, potassium hydroxide, and proton exchange membrane. A phosphoric acid fuel cell operates at about 160–220° C., and preferably at about 190–200° C. This type of fuel cell is currently being used for multi-megawatt utility power generation and for co-generation systems (i.e., combined heat and power generation) in the 50 to several hundred kilowatts range.
In contrast, proton exchange membrane fuel cells use a solid proton-conducting polymer membrane as the electrolyte. Typically, the polymer membrane must be maintained in a hydrated form during operation in order to prevent loss of ionic conduction which limits the operation temperature typically to about 70–120° C. depending on the operating pressure, and preferably below about 100° C. Proton exchange membrane fuel cells have a much higher power density than liquid electrolyte fuel cells (e.g., phosphoric acid), and can vary output quickly to meet shifts in power demand. Thus, they are suited for applications such as in automobiles and small scale residential power generation where quick startup is required.
Conventional fuel cells use hydrogen gas as the fuel. Pure hydrogen gas, however, is difficult and costly to supply. Thus, hydrogen gas is typically supplied to a fuel cell using a reformer, which steam-reforms methanol and water at 200–300° C. to a hydrogen-rich fuel gas containing carbon dioxide. Theoretically, the reformate gas consists of 75 vol % hydrogen and 25 vol % carbon dioxide. In practice, however, this gas also contains nitrogen, oxygen and, depending on the degree of purity, varying amounts of carbon monoxide (up to 1 vol %). This process is also complex, adds cost and has the potential for producing undesirable pollutants. The conversion of a liquid fuel directly into electricity would be desirable, as then a high storage density, system simplicity and retention of existing fueling infrastructure could be combined. In particular, methanol is an especially desirable fuel because it has a high energy density, a low cost and is produced from renewable resources. Thus, a relatively new type of fuel cell has been the subject of a great amount of interest—the direct methanol fuel cell. In a direct methanol fuel cell, the overall process that occurs is that methanol and oxygen react to form water and carbon dioxide and electricity, i.e., methanol combustion.
For the oxidation and reduction reactions in a fuel cell to proceed at useful rates, especially at operating temperatures below about 300° C., electrocatalyst materials are required at the electrodes. Initially, fuel cells used electrocatalysts made of a single metal, usually platinum (Pt), palladium (Pd), rhodium (Rh), iridium (Ir), osmium (Os), silver (Ag) or gold (Au) because they are able to withstand the corrosive environment—platinum being the most efficient and stable single-metal catalyst for fuel cells operating below about 300° C. While these elements were first used in solid form, later techniques were developed to disperse these metals over the surface of electrically conductive supports (e.g., carbon black) to increase the surface area of the catalyst which in turn increased the number of reactive sites leading to improved efficiency of the cell. Nevertheless, fuel cell performance typically declines over time because the presence of electrolyte, high temperatures and molecular oxygen dissolve the catalyst and/or sinter the dispersed catalyst by surface migration or dissolution/re-precipitation (see, e.g., U.S. Pat. No. 5,316,990).
Although platinum is a good catalyst, concentrations of carbon monoxide (CO) above about 10 ppm in the fuel can rapidly poison the catalyst surface. As a result, platinum is a poor catalyst if the fuel stream contains carbon monoxide (e.g., reformed-hydrogen gas typically exceeds 100 ppm). Liquid hydrocarbon-based fuels (e.g., methanol) present an even greater poisoning problem. Specifically, the surface of the platinum becomes blocked with the adsorbed intermediate, carbon monoxide (CO). It has been reported that H2O plays a key role in the removal of such poisoning species in accordance with the following reactions:Pt+CH3OH→Pt—CO+4H+4e−  (1)Pt+H2O→Pt—OH+H++e−  (2)Pt—CO+Pt—OH→2Pt+CO2+H++e−  (3).As indicated by the foregoing reactions, the methanol is adsorbed and partially oxidized by platinum on the surface of the electrode (2). Adsorbed OH, from the hydrolysis of water (3), reacts with the adsorbed CO to produce carbon dioxide and a proton. However, platinum does not adsorb H2O species well at the potentials fuel cell electrodes operate (e.g., 200 mV–1.5 V). As a result, step (3) is the slowest step in the sequence, limiting the rate of CO removal thereby poisoning the catalyst. This applies in particular to a Proton exchange membrane fuel cell which is especially sensitive to CO poisoning as a result of its low operating temperatures.
One technique for alleviating fuel cell performance reduction due to anode CO poisoning is to employ an anode electrocatalyst which is itself more poison tolerant, but which still functions as a hydrogen oxidation catalyst in the presence of carbon monoxide. It is known that the tolerance of platinum poisoning by carbon monoxide is improved by alloying the platinum with ruthenium, preferably compositions centered around 50:50 atomic ratio (see, e.g., D. Chu and S. Gillman, J. Electrochem. Soc. 1996, 143, 1685).
It has been reported that the success of the platinum-ruthenium catalyst alloys is based on the ability of ruthenium to adsorb H2O species at potentials where methanol is adsorbing on the platinum and facilitate the carbon monoxide removal reaction. This dual function, that is, to adsorb both reactants on the catalyst surface on adjacent metal sites, is known as the bifunctional mechanism in accordance with the following reaction:Pt—CO+Ru—OH→Pt+Ru+CO2+H++e−  (4).It has been suggested that having platinum and ruthenium in adjacent sites forms an active site on the catalyst surface where methanol is oxidized in a less poisoning manner because the adjacent metal atoms are more efficiently adsorbing the methanol and the water reactants.
Although knowledge of phase equilibria and heuristic bond strength/activity relationships provide some guidance in the search for more effective catalyst compositions, there is at present no way to calculate the chemical composition of different metals that will afford the best catalyst activity for the direct methanol-air fuel cell reaction. As such, the search continues for stable, CO poisoning resistant and less costly catalysts having increased electrochemical activities.